Effect of naloxone on perceived exertion and exercise capacity during maximal cycle ergometry

We assessed the effects of naloxone, an opioid antagonist, on exercise capacity in 13 men and 5 women (mean age = 30.1 yr, range = 21-35 yr) during a 25 W/min incremental cycle ergometer test to exhaustion on different days during familiarization trial and then after 30 mg (iv bolus) of naloxone or placebo (Pl) in a double-blind, crossover design. Minute ventilation ((V)over dotE), O-2 consumption ((V)over dotO(2)), CO2 production, and heart rate (HR) were monitored. Perceived exertion rating (0-10 scale) and venous samples for lactate were obtained each minute. Lactate and ventilatory thresholds were derived from lactate and gas-exchange data. Blood pressure was obtained before exercise, 5 min postinfusion, at maximum exercise, and 5 min postexercise. There were no control-Pl differences. The naloxone trial demonstrated decreased exercise time (96% Pl; P < 0.01), total cumulative work (96% Pl; P < 0.002), peak (V)over dotO(2) (94% Pl; P < 0.02), and HR (96% Pl; P < 0.01). Other variables were unchanged. HR and (V)over dotE were the same at the final common workload, but perceived exertion was higher (8.1 +/- 0.5 vs. 7.1 +/- 0.5) after naloxone than PI (P < 0.01). The threshold for effort perception amplification occurred at ∼60 ± 4% of Pl peak (V)over dotO(2). Thus we conclude that peak work capacity was limited by perceived exertion, which can be attenuated by endogenous opioids rather than by physiological limits.